When, in a watch, a component such as the crystal, or the back crystal, or the back cover, is required to be removable, this component is generally fitted onto the case middle or the case by means of a ring. This type of attachment is only possible when the case middle or case is made of metal. Indeed, if it is made of plastic, the material will creep under the stress exerted by the ring, and the component will fall off by itself.
It is known to use an additional metal piece to stiffen the case middle/case at the point of contact between the component and the case middle/case. However, the space available often prevents the addition of such a metal stiffener.
U.S. Pat. No. 3,386,239 in the name of SHIFFMAN discloses a watch case made of plastic material receiving a crystal mounted thanks to a bayonet fitting.